Role of GPU and CPU in terms of Gaming

[Sib Bachchan]

I have been playing various games for a long time now and on pack of every game I have seen mentioned in specification about the processor and the graphic card required for better performance of game. I still haven’t understood relation between working of CPU and GPU for a game. I mean I know that GPU is used for generating graphics, but then a video game is all graphics so what part does CPU play. It must be starting the game and other such work, but one thing I haven’t understood is that why every new game needs new processor. Execution of a game can be done even by older processors. Requirement of GPU is understood. I know that as the quality of graphics increase, so does the graphic card should increase in computing capacity. But still can’t understand how they work together to run a game. Can someone explain me about it?

[Kaesav]

It is your misconception that CPU is only needed to execute a game and for nothing else. If this was the case, even a Pentium 4 processor would have run games like Grand Theft Auto 4 with combination of high end graphic card. As the game becomes advanced in terms of graphics, so does its content gets advance. To load the proper content you see in the game, processor is needed. How else would the game know about which item to be initialized when? For example, a scene in a game, has character passing on the street while another character is shooting at a vehicle. As the user’s character comes near the car, an explosion takes place. In this scene, the GPU will render the car, characters, environment that may consist of buildings, other people and other objects depending on the theme. It will render the bullets and the explosion. A GPU will ass quality to all these effects and objects, but who will time the explosion? Who will make the character shoot car and no other object? And most important who will control the movements of your character? This all things are done by the CPU. It is the CPU which executes the codes developed by the game developers which does all these activities. So a game cannot just work using a graphic card but also need CPU. Asking about why games need a new CPU? As described in above scene, a CPU times the activities happening in the game, but in above scene the computer needs to maintain the simultaneous execution of various parts. Like while you are moving your character second character mustn’t stop shooting the car but continue his actions. At the same time if any other character exists in the scene, then he must not stop his activity. This shows that a CPU needs to maintain simultaneous execution of various activities. As the games develop, many such activities are introduced. In Grand Theft Auto 4, if you are in a mission, then you will have your character chasing your enemy. At the same time, the target will find new way to escape you. Simultaneously civilians will be walking around; they will be in large number. Many civilian cars will be in the scene that will be running on their path. The games will be sending you instructions related to mission, cops will be searching for illegal activities. So this execution of multiple activities to be executed smoothly, a CPU needs to be powerful.

[SanjoyChat]

In old games when CPU was not powerful, only limited set of actions would take place in a scene. Like, the enemy would only start attacking when your character came near. The far away characters would stand stiff even if it was possible for them to see your character. But nowadays, in new games like Hitman Blood Money, if you make even a small sound or enemy detects even a small threat they will start attacking. Here the enemy from far away will also detect these actions and will come to attack. So this shows that how much powerful a CPU needs to be. Game developed in these days have so many detailed actions as they are tried to be realistic, the processor needs to make various decisions all together. Thus you see new processor’s name mentioned in specifications of the game. GPU is needed to display the actions and decisions taken by the processor. The processor has its inbuilt GPU which does same work, but the problem is that if a CPU is given both activities, that is execute actions and also display them, then it might overload it. A processor can perform simple tasks like displaying text editors, web sites using the GPU built in it. It will even run basic games, but heavy games like Battlefield 3 would become too heavy for the processor for generating graphics as well as executing codes. Thus, a Graphic card is used, so that a processor can concentrate on processing the codes while graphic card handles content that is to be displayed.

[Hallo.genic]

Settings that you get in a game like resolution, quality of textures, field of view and anti-aliasing are controlled by graphic card. But the settings like amount of gore, or amount of traffic you need on streets of the in-game world or level of difficulty of game. When you change the resolution of game, the graphic card acts and fit the resolution according to the desired resolution. When the quality of textures is decreased, the graphic card will render fewer details from the graphic card, that is it will reduce the shine in the texture, it will reduce the resolution of texture and other details. Anti-aliasing deals with the smoothness of the object in a scene. Reducing it will show jagged edges on the sharp objects while smoothness is lost from rounded surfaces. Processor doesn’t deal much with it. When you reduce the difficulty of the game, this request is fulfilled by processor. It will decrease the amount of enemies that should be spawned. Processor will also reduce the level of weapons that enemies will carry and such other details. When the gore is reduced, process will not load the textures of blood and other such details. So this is how a CPU and GPU collaborate to make a game work properly.

[Kanesteyn]

As you have mentioned in your question, why CPU and GPU both require to be powerful, it also has another reason. Suppose only a powerful CPU is provided for a game to run, then it will execute the code and keep the data ready for displaying but the graphic card won’t be powerful enough to display it on monitor. Similarly if the graphic card is powerful and CPU is not, then graphic card will be ready to display content on screen but CPU must not even have executed code to decide what is to be displayed. To avoid such situation where a ‘bottleneck’ situation will be created, both CPU and graphic card must be equally powerful. To explain this concept of ‘bottle neck’ let me explain it to you using the relation between resolution and framerate.

Framerate and Resolution in a game are inversely proportional. For example, when I played Saints Row the Third on resolution of 1280 X 1024, the framerate was 40 fps, after reducing the resolution a bit, that is 1024 X 768, framerate went to 55 fps. So this shows that increased resolution decreases framerate. Resolution is controlled by graphic card, while framerate is controlled by CPU. The CPU generates enough data to be displayed in 55 frames per second. But since graphic card wants to display it in high resolution, it takes time to render it in that resolution and thus we get low framerate. In this case, CPU has generated enough data, but in order to increase its display quality graphic card was not accepting all of it. Thus, leading to low framerates. To avoid such situations, power of CPU and graphic card must be equally maintained. If not possible, then functionality of one of them must be compromised to make it equal to other. In many games, a CPU plays an important role as it uses multi-threading techniques. Resident Evil 5 and grand theft Auto 4 are games that use multi-threading techniques. Starcraft 2 is game that uses single thread technique. Thus, it will run even on somewhat older CPU.